A standard cigarette packing machine comprises a wrapping conveyor moving along a substantially circular wrapping path and having a plurality of recesses each of which is designed to receive a respective article together with a respective wrapping sheet; a plurality of folding stations successively arranged along the wrapping path, each folding station comprising folding means arranged to cooperate with the wrapping sheets to fold the same along pre-formed folding lines and around respective articles; a first of the folding stations is an insertion station arranged to insert each wrapping sheets inside a respective recess in the wrapping conveyor, and the last of the folding stations is an outlet or discharge station for discharging the packaged articles or the partially packaged articles from the wrapping conveyor.
Normally, the folding means in at least one of the folding station comprise one folding device of the type comprising a fixed member having a first flat surface; a movable member having a second flat surface; and an actuating device for activating the movable member to rotate the movable member between a rest position, in which said two surfaces are coplanar and define a supporting surface for a flat portion of the wrapping sheet to be folded, and a work position, in which said two surfaces form an angle of less than 180° so as to fold such portion of the wrapping sheet.
In known folding devices of the type described above, the fixed member and the movable member are connected by a hinge, the axis of which coincides with the axis of rotation of the movable member with respect to the fixed member. Since the supporting surface referred to must have no projections, the hinge must be located behind the supporting surface, with the result that the axis of rotation is actually located some distance from the supporting surface, and, as the second surface rotates with respect to the first, the line of intersection between the two surfaces moves crosswise to itself, and eventually reaches a final position when the movable member reaches said work position.
Consequently, when a flat portion of the wrapping sheet is placed on the supporting surface, rotation of the movable member about said axis of rotation results in the wrapping sheet sliding along the second surface, and in various other phenomena, which vary depending on whether or not the wrapping sheet has a preformed fold line.
In the absence of a preformed fold line, rotation of the movable member produces, along the wrapping sheet, an elbow located at the line of intersection and which moves with it gradually reducing its radius of curvature and so increasing the compressive stress within the wrapping sheet. For a given wrapping sheet thickness and given rotation speeds of the movable member, such stress may exceed the critical combined bending and compressive stress of the wrapping sheet, thus resulting, due to collapse of the wrapping sheet, in the formation of false folds before the line of intersection reaches the final position.
If there is a preformed fold line, collapse of the wrapping sheet always occurs along the preformed fold line, though the wrapping sheet is still forced to slide along the second surface, possibly undergoing damage and forming wrinkles.
U.S. Pat. No. 4,055,066A1 relates to an automatic apparatus for forming bends in sheet metal or any other deformable material, and particularly sheet metal of large dimensions. The automatic apparatus includes a bending unit with a fixed frame, female holding means for clamping the sheet in a vertical alignment, mobile tables for bending the sheet, which tables rotate on an axis parallel to the longitudinal axis of the apparatus on respective intermediary supports; each intermediary support is independently moveable on the fixed frame from one position corresponding to a retracted position of the table to a second position which is a working position of the table. The intermediary supports and tables are independently operable.
FR2019941A discloses an apparatus for forming bends in sheet metal of large dimensions, wherein a fixed block and a movable block activated by an articulated parallelogram define respective contiguous flat surfaces blending along a fold line; the actuating device has two connecting rods connected by cranks hinged to the fixed block to rotate, with respect to the fixed block, about respective axes parallel to the fold line, and the movable block is hinged to the two connecting rods, and defines a further crank rotating, with respect to the fixed block, about an axis coincident with the fold line.